Hmm.
I've just spent about 30 minutes studying a map of Essex, looking at various possibilities and checking out websites. Unfortunately, most of the places I've looked at (e.g. the three railway museums -Colne Valley, Mangapps & East Angian Railway) can't even be bothered to mention 'accessibility' on their web pages. You seem to have already visited most of the other interesting places which are wheelchair-friendly.
My best offering might be to spend an hour or two at Mole Hall Wildlife Park before crossing just 3 miles over the county boundary and visiting the Duxford Air Museum.
Mole Hall seems to be wheelchair-friendly. There are slightly reduced rates for disabled visitors and the interesting-sounding (if somewhat wet!) water maze is specifically described as 'wheel-chair friendly').
http://www.molehall.co.uk/
Duxford is largely wheelchair accessible:
http://duxford.iwm.org.uk/server/show/nav.620
Home page:
http://duxford.iwm.org.uk/
Chris